Beam forming antenna



Aug.. 5, 1958 A. CHL/WIN BEAN FORMING ANTENNA Filed Dec. l5. 1953 IJIM l w u United States Patent O BEAM FORMING ANTENNA Alvin Chlavin, Los Angeles, CalifL, assigner, by mesne assignments, to Hughes Aircraft Company, a corporation of Delaware Application December 15, 1953,v Serial No. 398,263 4 Claims. (Cl. 343-775) This invention relates. generally to microwave radiators and receptors and, more particularly, to an antenna for producing a beam which -is substantially circular in cross section.

For etlicient use of a reflector having a circular aperture, such as a paraboloid,y it is required that an illuminating beam. of substantially circular cross section be provided. Only in this way can the gain of a circular reflector be maximized. Since conventionalv antennas or primary radiators of the type most suitable for illuminating a parabolic reflector doY not produce such a beam, considerably less than optimum aperture eiliciency has been attained in the past.

In addition to the factor of etlciency, conventional primary radiators of this type are somewhat unsatisfactory for use in circularly polarized wave transmission systems. This is because the beam formed by the parabolic rellector becomes elliptically polarized in angular regions remote from the beam axis. owing to the non-circular illnmination provided.

It is an object of this invention, therefore, to provide an antenna which forms a beam` having a substantially circular cross section and which is suitable for illuminating a circular reflector.

lt is another object of this invention to provide an antenna having a substantially constant impedance over a relatively wide range of operating frequencies.

It is a further object to provide an antenna suitable for illuminating a circular rellector without obstructing appreciably the beam which is formed thereby.

The antenna feed according to this invention comprises, in effect, two distinct radiating elements having complementary radiation patterns. One of these elements is a dipole which has a radiation pattern that is broad in a plane perpendicular to the electric vector E and relatively narrow in a plane parallel thereto. The other of these elements is the open end of a coaxial transmission line carrying TEM mode electromagnetic wave energy. This latter element has a radiation pattern which is broad in a plane parallel to the electric vector E, and relatively narrow in a plane perpendicular thereto. By combining these elements, according to the invention, a beam with a substantially circular cross section may be readily obtained.

The novel features of this invention, together with further objects and advantages thereof, will be better understood when considered in connection with the accompanying drawing in which:

Fig. l is a perspective View of an antenna structure in accordance with this invention;

Fig. 2 is a cross-sectional view of the antenna structure of Fig. l;

Fig. 3 is a graph wherein the radiation pattern of the antenna structure of Fig. 1 is illustrated;

Fig. 4 is a graph representing the impedance of the antenna structure of Fig. l over a band of operating frequencies; and

Pig. 5 is a perspective view of a modification of the 2,846,679 Patented Aug. 5, 1958 ICC 2 antenna structure of Fig. l according to this invention.

Referring now to the. drawing wherein like elements are designated -by the same reference characters and,v morey particularly,v to Figs. l and 2, the antenna of the invention is seen to include a length of rectangular hollow uniconductor waveguide 11 which is coupled to a dipole 12. Waveguide 11 is lformed in three sections 14, 15, and 16 ofvarying narrow dimension, the narrowest section 16 being. closed so as to provide a short circuit at the end of the waveguide. Section 14, which is the section remote from the short circuit, has cross sectional or transverse dimensions in the ratio of approximately 2:1. That is, the Width a yof section 14 is approximately twice its height b. Section 16, on the other hand, is approximately half` as high as section 14, but of the same width. A smooth transition between sections 14 and I6 is provided by section 15 whose narrow dimension or height gradually tapers from the height of section 1'4 toy the height of section 16. The purpose of this arrangement is to facilitate the connection of the antenna to an external waveguide of conventional dimensions.

To couple. dipole 12 to waveguide 11, there are formed in the respective broad walls of section 16 a pair of apertures equidistant from the narrow walls. Dipole element 12 extends across section 16 and projects through. each of the apertures an equal amount, thev axis of dipole 12 intersecting at right angles the longitudinal axis of section 16. To support dipole element 12 in this position, there is included a dielectric element 17 interposed between the broad walls of section 16. Element 17V is also provided with an aperture aligned with the apertures in the broad walls of section 16, thereby to receive dipole element 12.

Finally, there is provided a reflector 13 shaped in the form of a right circular cylinder having Ione of its ends closed and the other of its ends open in proximity to dipole element 12. The axis of the cylinder coincides with the longitudinal axis of section 16 so that, in elect, section 16 and retlector 13 form a length `of short circuited coaxialtransmission line having a rectangular inner conductor. This transmission line may be readily adapted to support TEM mode wave energyV by properly dimensioning section 16 and reflector 13. Accordingly, a portion of the energy radiated by dipole element 12 will then excite TEM mode wave energy in rellector 13 for re-- radiation into space. Since the pattern of the reradiated energy is complementary to the dipole radiation pattern in that the former is -broader in the plane of dipole element 12 whereas the latter is broader in a plane perpendicular thereto, a correct apportionment of the energy in each pattern yields the required beam of circular cross section.

By way of illustration, the following dimensions were found to be most satisfactory to produce this result for operating frequencies in the range between 8,500 and 9,50() megacycles. Thus, dimension A, representing the inside diameter of reector 13 is 1.350 inches, and dimension B, the length of reector 13, is .350 inch. Similarly, dipole 12 is approximately .900 inch in length, C, and the axis of dipole 12 is spaced a distance D, or .062 inch from the aperture of reflector 13. To complete the illustrative proportions of the antenna, a height E of .200 inch for section 16 is provided, section 16 being short circuited at a distance F or .545 inch from dipole element 12.

Fig. 3 illustrates the radiation pattern of the antenna according to this invention in two orthogonal planes passing through the axis of waveguide 11. In particular, line 18 represents the pattern in the plane parallel to the dipole axis, generally referred to as the E plane, whereas the dotted line 19 represents the radiation pattern in the plane perpendicular to the dipole axis, generally referred to as the H plane. Although the radiation patterns in these planes are shown only; for angles between and 180 degrees referred to the Yaxis of waveguide 11, it will befapparent to those skilled in the art that thepatterns in'the remaining 180 degree sector are substantially the same as those shown.

As illustrated in Fig. 3, the E and the H plane patterns.

are substantially identical, particularly in the range between zero and 100 degrees. Although at angles greater than 100 degrees there is some divergence between lines 18 and 19, very little energy is radiated at these angles. Accordingly, most of the energy radiated by the antenna of this invention is concentrated in a relatively broad beam of substantially circular cross section as is required for efficient illumination of a parabolicreector. It is also signicant that the front-to-back ratio of the antenna is aminimum for any given size of reflector 13.

'Referring now to Fig. 4, the impedance characteristic ofthe antenna is illustrated, in terms of standing wave ratio in waveguide section 14. As seen fromFig. 4, the voltage standing wave ratio does not exceed 1.5 for operating frequencies in the range .between 8,500 and 9,500 megacycles. It is apparent, therefore, that the antenna of this invention'is well suited for operation over a relatively broad band of frequencies.

Fig. illustrates a modification of the antenna structure of Figs. l and 2 wherein a conically shaped reflector 26 is utilized. Rather than rectangular waveguide, the modified antenna includes a length of coaxial transmission line 21 lled with dielectric material, the inner conductor 22 and outer conductor 23 of line 21 being dimensioned to transmit wave energy in the TEM mode only. Dipole 24 is 'joined to inner conductor 22 at right angles to its longitudinal axis and projects through apertures provided inouter conductor 23. Terminating transmission line 21 in a short circuit is the conical reflector 26, which is coaxial with transmission line 21. As is apparent, this antenna structure, like that of Figs. l and 2 is adapted to ,transmit and receive linearly polarized waves. For circularly polarized wave transmission, a second dipole at right angles to dipole 24 would be included.

Provided that reector 26 is properly dimensioned, there is again formed a beam of circular cross section, owing to the complementary radiation patterns of reliector 26 and dipole 24. In the 9,000 rnegacycle operating frequency range, for example, rellector 26 should have a diameter of approximately .940 inches and a length of approximately .670 inch. Similarly, the vertex of reflector 26 should be spaced from the center of dipole 24 a distance of approximately .350 inch, Reflector 26 of Fig. 5 is, of course, interchangeableY with rellector 13 of Figs. 1 and 2 with slight dimensional modifications.

What is claimed as new is:

l. A microwave antenna having a radiation pattern representative of a beam'which is substantially Vcircular in cross section, said antenna comprising a length of rectangular hollow uniconductor waveguide having broad and narrow walls, each of the broad walls of said waveguide being provided with an'aperture positioned substantially equidistant from the narrow walls, a dipole element extending across said waveguide and projecting through said apertures, the axis of said dipole element being substantially perpendicular to the longitudinal axis of said waveguide, and a reflectorshaped in the form of a right circular cylinder having one of its ends closed,

said reectortbeing disposed with its longitudinal axis eoinciding with'the longitudinal axis of said waveguide and its open end facing said dipole element.

2. A microwave antenna according'to kclaim l wherein the diameter of said reflector is approximately one wavelength in free space and the longitudinal dimension of said reflector is approximately one quarter wavelength in free space.

3. An antenna feed according to claim 2 wherein the narrow transverse dimension of said waveguide is approximately 0.15 wavelength in free space,

4. An antenna feed according to claim 1 wherein the narrow transverse dimension of said waveguide is approximately 0.15 wavelength in free space.

References Cited in the file of this patent UNITED STATES PATENTSY Kline May 6, 1952 

